The xeroprinting process employs a printing plate, commonly referred to as a "master", made by creating a pattern of insulating material, i.e., an image, on the surface of a grounded conductive substrate. In the xeroprinting process, an electrostatic charge is applied to the surface of the master, e.g., by corona discharge. The portion of the master bearing the insulating material retains the charge, while the charge on the remainder of the master is discharged through the grounded conductive substrate. Thus, a latent image of electrostatic charge is formed on the insulating material, the image subsequently being developed with either oppositely charged particles commonly referred to as "toner" or liquid electrostatic developers. The toner is then transferred, e.g., by electrostatic or other means, to another surface, e.g., paper or polymeric film, where it is fused, i.e., "fixed", to reproduce the image of the master. Since the image on the master is permanent multiple copies can be made by repeating the charging, toning and transfer steps.
Riesenfeld et al. U.S. Pat. No. 4,732,831 discloses an improved xeroprinting process that employs a master having a photopolymerizable or photohardenable coating on a conducting substrate. The coating contains an organic polymeric binder, an ethylenically unsaturated monomer, and a photoinitiator system. When the master is exposed to the desired pattern of actinic radiation (i.e., light of a suitable wavelength), exposed regions of the coating polymerize and exhibit a significantly higher electrical resistance than unexposed regions. Thus, when the master is subsequently used in the xeroprinting process, the polymerized regions will hold an electrical charge, which is developed with toner, while the unpolymerized regions discharge to ground through the conductive backing and therefore do not attract the toner.
It has been found that the electrostatic properties of the photopolymerizable masters change considerably with small variations in ambient temperature around room temperature. These changes in electrical properties with ambient temperature and humidity degrade image quality and dot gain. It has also been found that when blends of binders of significantly different Tg's are incorporated into formulations the environmental stability of the photopolymer electrostatic masters improve noticeably. In general, a high Tg/high resistivity binder such as poly(styrene/methyl methacrylate) (70:30) was mixed with a lower Tg binder, e.g., high molecular weight Elvacite.RTM. 2042 or Elvacite.RTM. 2045. Multiple binders were introduced to broaden the glass transition of the exposed and unexposed regions. They improved the overall master performance by reducing the variation of viscosity, which, in turn, is associated with the variations in discharge rate, with temperature fluctuations. At high temperatures, the unexposed master discharges more rapidly and, as a result the dot gain decreases and ultimately the highlight dots are lost. In contrast, a decrease in discharge rate at low temperatures is associated with loss of shadows dots and increased dot gain. Although multiple binder systems noticeably improved the environmental performance, especially in the range of 30% .ltoreq.relative humidity, .ltoreq.60% and 60.degree. F. (15.6.degree. C.) .ltoreq.temperature .ltoreq.80.degree. F. (26.7.degree. C.), light scattering adversely affected the dot range and exposure latitude achievable with single binder systems.
In general, most polymeric binders of reasonable molecular weight are incompatible with one another. The result of this is that at typical concentrations one observes phase separation of the two binders within the mixture. A standard method of detecting phase separation is the cloud point as a function of temperature or concentration. The cloud point is where there is formed small volume elements rich in one polymer and poor in the other along with other volume elements of opposite nature. The dimensions of these volume elements are typically about the wavelength of light. These small regions of fluctuating dielectric constant (or index of refraction) result in a large amount of scattering and hence the cloudy nature of the mixture.
Haziness and dot range (or lack thereof) are a direct result of the phase separation or binder incompatibility. In a clear photopolymerizable system the incident photon is absorbed within a distance of about 1/.lambda. in the direction of the incident light wherein .lambda. is the wavelength of the incident light. In the case of multiple binders light scatters at the interfaces of the two phases and the photon re-radiates in any angle before polymerization occurs. As a result, although the light travels the same distance of 1/.lambda., the direction has changed and polymerization can occur in regions where it is not desired.
It has now been found that a photohardenable electrostatic master having improved resolution, wherein the dot range of reproduced halftone dots and exposure latitude are controlled, can be made by introducing into the photohardenable composition forming the photohardenable layer a polymerization inhibitor of the type and in the amount set out below.